Formation flight can be described as an arrangement of two or more aircraft flying together in a fixed pattern as a cohesive group. Different types of aircraft regularly can be flown in formation. One example of a formation flight is aerial refueling, where a receiver aircraft flies behind and below a tanker aircraft. In some of these formations, the aircraft are sufficiently close to one another that their wakes affect the aerodynamic characteristics of each other. This situation is sometimes referred to as “close formation flight”.
Close formation flight is attractive because of its potential to significantly reduce the aerodynamic drag and increase lift for the aircraft in formation. These effects in turn can lower power required for propulsion, reduce fuel consumption, and increase aircraft endurance, flight range, and payload.
While there are definite aerodynamic benefits of such formation flight, it has not been used in practice so far due to difficulties in flight control in close formation. Furthermore, the close proximity of the aircraft presents an unacceptably high risk of collision for most applications.
Close formation flight can be used for aerodynamic drag reduction, with a follower aircraft flying in the upwash generated by a leader aircraft. However, it has been very difficult for pilots on piloted aircraft and autopilots on unmanned airborne vehicles (UAV) to maintain proper positions in the formation for extended periods of time. In both cases, manned and unmanned aircraft, special automated control systems are required. Such systems must be able to determine relative locations of the aircraft and their trailing vortices to a very high degree of accuracy, in order to produce and sustain a close formation.
Wake turbulence is typically generated in the form of vortices trailing behind aircraft wing tips and other lifting surfaces. The pair of vortices generated by each aircraft is the result of lift being generated by the wings and air rotating around the wingtips from the high pressure regions at the bottom of the wing to the low pressure regions at the top of the wing.
Generally, these vortices are considered dangerous to other aircraft, particularly to those positioned directly behind within the wake turbulence. The wingtip vortices generated by a leading aircraft typically negatively affect the flight of trailing aircraft, by disrupting its aerodynamics, flight control capabilities and potentially damaging the aircraft or its cargo and injuring the crew. This makes manual flight control in close formation very difficult and challenging. As a result, conventional autopilot systems prevent close formation flight, by avoiding areas with wake turbulence.
Therefore, the inventors believe there is a need for an advanced adaptive flight control system with capabilities to provide reliable and accurate onboard flight control for aircraft in close formations. Such a system would enable multiple aircraft, both manned and unmanned, to produce and maintain close formation flight for extended time and thus achieve substantial benefits in aerodynamics performance outlined above.
Proposed solutions for such a system so far have been limited in their accuracy and efficacy. Some flight control systems are equipped to estimate the position of wingtip vortices trailing a leading aircraft, and control the flight characteristics of trailing aircraft to avoid the vortices. The position of a wingtip vortex relative to a trailing aircraft is estimated based on the flight characteristics of the leading aircraft and an estimate of the wind generated by the trailing aircraft.
Proposed close formation flight systems, as a rule, do not account for the effects of winds and drift on the wingtip vortices. The wingtip vortices, however, may move under the influence of winds and shift their position unpredictably between the leading and trailing aircraft. Because wingtip vortices cannot be directly visualized, the uncertainty in their position makes close formation flight not only challenging, but often impossible.
Older systems for formation flight control typically implemented a gradient peak-seeking approach to move the objects relative to each other to maximize or minimize a desired metric, i.e., fuel consumption. This approach uses a dither signal to determine a change in relative position to improve the metric. The change is effected, the results analyzed, and the position further updated once again using a dither signal to continually improve the metric. This gradient approach to peak-seeking may eventually position the aircraft close to the desired relative position in an ideal situation. However, such an approach is sluggish, time-consuming and unresponsive, so that in fast-changing conditions it becomes ineffective.
Some conventional formation flight control systems attempt to estimate the position of a wingtip vortex and control the position of a trailing aircraft relative to the estimated position. An inaccurate estimate of the vortex position leads to inaccurate relative positioning of the aircraft in formation. In addition, existing formation flight control systems fail to adequately account for vortex-induced aerodynamic effects acting on the aircraft.
Thus, the inventors have provided embodiments of improved apparatus, systems, and methods for close formation flight.